tildify: add `tildify-space' and `tildify-mode'

tildify: add `tildify-space' and `tildify-mode'

* lisp/textmodes/tildify.el (tildify-space): A new function
which can be used as a `post-self-insert-hook' to automatically
convert spaces into hard spaces.
(tildify-space-pattern): A new variable specifying pattern where
`tildify-space' should take effect.
(tildify-space-predicates): A new variable specifying list of
predicate functions that all must return non-nil for
`tildify-space' to take effect.
(tildify-space-region-predicate): A new functions meant to be
used as a predicate in `tildify-space-predicates' list.
(tildify-mode): A new minor mode enabling `tildify-space' as a
`post-self-insert-hook'

* tests/automated/tildify-tests.el (tildify-space-test--test):
A new helper function for testing `tildify-space' function.
(tildify-space-test-html, tildify-space-test-html-nbsp)
(tildify-space-test-xml, tildify-space-test-tex): New tests for
`tildify-space' function.